The present invention relates to a novel aluminoborosilicate containing an alkaline earth metal, a method for the preparation thereof and a method for the catalytic preparation of a lower olefin using the same as the catalyst. More particularly, the invention relates to a novel microcrystalline zeolite having a specific crystalline structure useful as a catalyst for the gas-phase preparation of lower olefins, such as ethylene and propylene an efficient method for the preparation of such a novel microcrystalline zeolite and an efficient method for the preparation of a lower olefin using the microcrystalline zeolite as the catalyst.
As is known, one of the world-wide issues of importance in recent years is the problem of exhaustion of the petroleum resources and this problem is especially serious in countries having no or little source of petroleum such as Japan. Accordingly, it is eagerly desired to develop an efficient method for the utilization of coals, natural gases and the like as a substitute of petroleum. For example, it would be very desirable to establish an industrially feasible method for the synthetic preparation of olefins, paraffins, aromatics and the like from methane and carbon monoxide via methyl alcohol. Most of such processes are performed by the catalytic gas-phase reaction using a solid catalyst and various types of zeolites or crystalline aluminosilicates and related materials have been proposed as a catalyst therefor.
A variety of zeolitic crystalline aluminosilicates occur in nature but most of the catalytically useful zeolites are synthetically prepared. Synthetically prepared zeolites have a well-controlled crystalline structure with a large number of interstices and tunnel-like pores therein to serve as a so-called molecular sieve capable of adsorbing only molecules of adsorbates having a dimension smaller than a certain upper limit. The configuration and dimension of the interstices and pores are determined by the manner in which the units of SiO.sub.2 and Al.sub.2 O.sub.3 in the crystalline structure are covalently bonded together possessing the oxygen atoms in common. The electric neutrality of the tetrahedron containing the aluminum atom is maintained usually with alkali metal ions or, in particular, ions of sodium and/or potassium.
Zeolitic crystalline aluminosilicates are usually prepared by the hydrothermal reaction under normal or superatmospheric pressure of an aqueous reaction mixture containing the source materials of silica, alumina and alkali metal ions. The basic compound added to the reaction mixture may be an organic nitrogen compound or an organic phosphorus compound so as to give a zeolite product having specifically modified adsorption characteristics and catalytic activity. For example, the ZSM-type zeolites are prepared by use of an organic basic compound such as tetraalkyl ammonium compounds, tetraalkyl phosphonium compounds, pyrrolidine, ethylene diamine, choline and the like and are highlighted in recent years in respect of their very unique adsorptivity and catalytic activity. In particular, the ZSM-5 zeolite has pores of a medium pore diameter of 5 to 6 .ANG. to exhibit specific adsorptivity by which linearly chained or slightly branched hydrocarbon molecules can be adsorbed thereon but highly branched hydrocarbon molecules cannot be adsorbed. It is reported in Japanese Patent Kokai No. 52-43800 that the ZSM-5 type zeolite can be synthesized by the hydrothermal reaction of an aqueous reaction mixture containing the source materials of silica, alumina and alkali metal ions in combination with tetra(n-propyl) ammonium compound as the organic base.
The synthetic process for the preparation of hydrocarbons from methyl alcohol and/or dimethyl ether as the starting material is under extensive and intensive investigations in recent years. The reaction is usually performed by use of a so-called solid acid catalyst and the subject matters of many patent publications have been various kinds of zeolites, heteropolyacids and the like useful therefor. For example, the above mentioned ZSM-5 type zeolite exhibits excellent catalytic performance in the synthetic process for the preparation of hydrocarbons mainly composed of the gasoline fraction having up to 10 carbon atoms in a molecule from methyl alcohol as the starting material in respect of the stability of the catalytic activity and the catalyst life. A problem in the use of this type of the solid catalyst is the relatively low selectivity for the formation of lower olefins such as ethylene and propylene. On the other hand, a high selectivity for the formation of ethylene and propylene in the same reaction as above can be obtained by using a zeolite of another type ZSM-34 as the catalyst although the zeolite of this type is practically not feasible because of the rapid decrease of the catalytic activity at high temperatures due to the deposition of carbonaceous materials thereon.
Accordingly, it is a very important problem in this technical field of chemical industry to provide a solid catalyst capable of exhibiting excellent catalytic performance including a high selectivity for the formation of lower olefins and a long catalytic life even at high temperatures without decrease in the catalytic activity due to the deposition of carbonaceous materials.